In recent years, less expensive video cameras with high sensitivity and resolution are available, and a region in a wide range such as sea can be imaged by a practical number of cameras, and the image of an object can be captured.
A technique is known, which detects a boat which has intruded into a surveillance area at sea, without being affected by waves (see, for example, Patent Literatures 1 to 6).
It is to be noted that a known technique related to the present invention estimates a phase velocity, a period, and a wave height of waves on the sea surface from camera images (see, for example, Non Patent Literatures 2 and 3).